Add support for FieldRva to EnC delta

[tmat]

As discussed on dotnet/runtime#112446, we append the field data to the IL stream and use existing RVA offset interpretation, except relative to the start of the IL stream.

Adds a new EnC capability AddFieldRva. If this capability is returned from the runtime the IDE instructs the compiler to use FieldRVA table to emit array initializers, stackalloc initializers, UTF8 strings and other features that rely on FieldRVA data in regular (non-EnC) emit.

Allow Hot Reload to emit string literals when the limit on #UserString heap is reached.

Moves emit of data section string literals down to ILBuilder, so that we can handle

1. all string literals, not just those appearing in expressions
2. heap overflow in EnC by tracking heap capacity.

Fixes #69480
Implements #78035
Depends on dotnet/runtime#112446

[tmat]

@dotnet/roslyn-compiler @cston @jjonescz ptal
@DustinCampbell fyi

[DustinCampbell]

Looks like this'll need API review?

[jaredpar]

@cston PTAL