Add schema cache

[nirinchev]

Description

Fixes #2194

This will not have massive performance effect for most users as we expect most people to just use the default schema, which is already cached. But for people who do set ObjectClasses, it should speed things up nicely.

Benchmarks

Before the change

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
CreateSchema	3.897 ms	0.0313 ms	0.0261 ms	78.1250	7.8125	-	859 KB
OpenRealm	16.567 ms	0.2680 ms	0.2375 ms	62.5000	-	-	898 KB

After the change

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
CreateSchema	2.128 us	0.0090 us	0.0075 us	0.1793	-	-	2 KB
OpenRealm	12,254.235 us	230.4506 us	215.5636 us	-	-	-	40 KB

TODO

• Changelog entry
• Tests (if applicable)

[papafe]

Looks good