feat: add support for maps

[anthony-swirldslabs]

Description:
Introducing support for map fields in PBJ.

I apologize for the size of the PR but it would be close to impossible to break it up into reasonable pieces because all the code is deeply interconnected and needs to be updated all at once when adding a new field type.

Short summary:

1. The compiler is updated to generate Protobuf and JSON models/codecs for messages with map fields.
2. In models, the maps are present as instances of a new PbjMap class that implements an immutable map and provides access to a list of sorted keys to support deterministic iteration over the map entries.
3. Per the spec and the implementation of maps in protoc, they are... strange. In general, on the wire they're presented as repeated entry messages with key/value fields. However, unlike normal protobuf messages, the entries always serialize key and value values, even when the values are "default" (e.g. "" for a string, or 0 for an int, etc.) This peculiarity required quite a bit of refactoring in the codec generator. All those skipDefault-related changes are here to support just this.

Related issue(s):

Fixes #254

Notes for reviewer:

1. All the tests, including the integration and fuzz tests pass.
2. A new test model map.proto is added to test various maps.

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[github-actions]

JUnit Test Report

     48 files  ±0       48 suites  ±0   4m 20s ⏱️ + 1m 40s
1 138 tests ±0  1 137 ✔️ ±0    1 💤 ±0  0 ❌ ±0 
6 993 runs  ±0  6 976 ✔️ ±0  17 💤 ±0  0 ❌ ±0 

Results for commit a0fc753. ± Comparison against base commit 6da76df.

♻️ This comment has been updated with latest results.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
❌ -43.15% (target: -1.00%)	✅ 0.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (6da76df)	4361	1977	45.33%
Head commit (a0fc753)	2151 (-2210)	47 (-1930)	2.19% (-43.15%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#258)	284	0	0.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[github-actions]

Integration Test Report

     269 files   -        5       269 suites   - 5   8m 3s ⏱️ + 1m 57s
90 327 tests  - 6 327  90 327 ✔️  - 6 327  0 💤 ±0  0 ❌ ±0 
90 555 runs   - 6 327  90 555 ✔️  - 6 327  0 💤 ±0  0 ❌ ±0 

Results for commit a0fc753. ± Comparison against base commit 6da76df.

♻️ This comment has been updated with latest results.

[jasperpotts]

Impressive work, that was a lot of work :-)