Add JsonEnumMemberNameAttribute.

[eiriktsarpalis]

Makes the following changes:

1. Implements Support customizing enum member names in System.Text.Json #74385.
2. Moves to using allocation-free enum parsing via the new alternate lookup types.
3. Adds handling and tests for char-backed enum types that are possible in F#.

Fix #74385. Pending API review.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.

[jozkee]

Otherwise; LGTM.

[eiriktsarpalis]

I've reworked the parsing implementation and refined the casing semantics and identifier validation logics. The implementation also improves performance substantially as this benchmark shows:

Method	Branch	_value	Mean	Error	StdDev	Median	Min	Max	Ratio	MannWhitney(3%)	Gen0	Allocated	Alloc Ratio
Serialize	main	Default	40.85 ns	0.469 ns	0.439 ns	40.85 ns	40.29 ns	41.58 ns	1.00	Base	0.0007	-	NA
Serialize	PR	Default	29.02 ns	0.097 ns	0.081 ns	28.99 ns	28.91 ns	29.16 ns	0.71	Faster	-	-	NA
Deserialize	main	Default	82.40 ns	0.232 ns	0.194 ns	82.32 ns	82.17 ns	82.77 ns	1.00	Base	-	-	NA
Deserialize	PR	Default	81.06 ns	0.146 ns	0.137 ns	81.03 ns	80.86 ns	81.29 ns	0.98	Same	-	-	NA
SerializeDictionary	main	Default	195.27 ns	1.157 ns	1.082 ns	195.12 ns	193.91 ns	197.35 ns	1.00	Base	0.0036	112 B	1.00
SerializeDictionary	PR	Default	153.45 ns	1.738 ns	1.626 ns	152.94 ns	151.90 ns	156.54 ns	0.79	Faster	0.0021	-	0.00
Serialize	main	Insta(...)ublic [27]	39.51 ns	0.388 ns	0.344 ns	39.45 ns	39.09 ns	40.18 ns	1.00	Base	0.0008	24 B	1.00
Serialize	PR	Insta(...)ublic [27]	34.98 ns	0.171 ns	0.151 ns	34.93 ns	34.84 ns	35.38 ns	0.89	Faster	-	-	0.00
Deserialize	main	Insta(...)ublic [27]	130.20 ns	0.413 ns	0.366 ns	130.10 ns	129.73 ns	130.92 ns	1.00	Base	-	-	NA
Deserialize	PR	Insta(...)ublic [27]	94.75 ns	1.139 ns	1.010 ns	94.93 ns	93.47 ns	96.49 ns	0.73	Faster	-	-	NA
SerializeDictionary	main	Insta(...)ublic [27]	456.33 ns	5.461 ns	5.108 ns	457.01 ns	448.09 ns	466.25 ns	1.00	Base	0.0196	528 B	1.00
SerializeDictionary	PR	Insta(...)ublic [27]	333.09 ns	1.271 ns	1.127 ns	332.90 ns	331.50 ns	335.34 ns	0.73	Faster	0.0062	-	0.00
Serialize	main	2147483647	105.37 ns	1.391 ns	1.233 ns	105.35 ns	102.84 ns	107.50 ns	1.00	Base	0.0052	-	NA
Serialize	PR	2147483647	52.69 ns	0.148 ns	0.124 ns	52.68 ns	52.51 ns	52.96 ns	0.50	Faster	-	-	NA
Deserialize	main	2147483647	71.29 ns	0.125 ns	0.098 ns	71.27 ns	71.15 ns	71.50 ns	1.00	Base	-	-	NA
Deserialize	PR	2147483647	71.19 ns	0.156 ns	0.130 ns	71.19 ns	70.94 ns	71.46 ns	1.00	Same	-	-	NA
SerializeDictionary	main	2147483647	234.79 ns	2.926 ns	2.594 ns	234.70 ns	230.67 ns	239.88 ns	1.00	Base	0.0067	192 B	1.00
SerializeDictionary	PR	2147483647	118.84 ns	0.302 ns	0.282 ns	118.79 ns	118.47 ns	119.37 ns	0.51	Faster	-	-	0.00