Optimize ResourceContainer/AssemblyLoadEventHandler methods, remove allocs (#9822)

* Rename _registeredResourceManagers -> s_registeredResourceManagers

* Use StringComparer.OrdinalIgnoreCase instead of name lowering

* Avoid double dictionary lookup in UpdateCachedRMW

* Use proper lookup in GetResourceManagerWrapper (value can never be null)

* Use proper variable names, set as readonly variables that are never set again

* Mark UpdateCachedRMW and GetResourceManagerWrapper as static functions

* Use string.create for key creation with initial buffer

* Save 100ns by using faster byte2hex function

* Save additional 200 bytes or more in allocations

* Formatting changes only

* Shave off 300ns per invocation, allocation-free in 99.9% cases

* Seal the class, remove redundant usings, final formatting changes/comments only

* GetResourceManagerWrapper is mostly just using key for lookup, no reason for string

* Revert the StringComparer.OrdinalIgnoreCase usage because of Span<char> buffer

* Fix original comment typo

* Fix a few more comments, revert list separator

* Fix usings post-merge

Author: h3xds1nz