Add Marshalers for profiling signal type

[jhalliday]

The third step towards support for the new experimental profiling OTLP signal type, following on from the data model interfaces and implementation.

These marshaler classes are somewhat verbose and very boilerplatey, but should not contain any surprises, being patterned largely on the existing marshalers for other signal types. The code is tedious to review, particularly to check the field names and data types correspond correctly to the proto file. I think I got all the cut and paste errors out, but then again I thought that on two previous occasions whilst checking it and proved myself wrong both times. Should probably have written a codegen tool instead, but hey, hindsight...

The classes here cover the payload, pprofextended.proto The PR is already a bit big, so the ones for the envelope, profiles.proto, will follow later.

The utility classes in common are extended with some additional methods to support data types used by profiling but not other signal types.

For the first time we actually get some test coverage on profiling code. The tests for the marshalers use the data model interface and impl, so those get exercised too. May even have gone a bit too far - the code and tests were written starting from the leaf nodes, whereas just testing serialization of the root type would have transitively tested the rest. I'm leaving all the tests in for now, but they could be curated a little to reduce duplication if desired.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 56 lines in your changes missing coverage. Please review.

Project coverage is 90.31%. Comparing base (09c7c4a) to head (2067223).
Report is 19 commits behind head on main.

Files	Patch %	Lines
...elemetry/exporter/internal/marshal/Serializer.java	0.00%	28 Missing ⚠️
...metry/exporter/internal/marshal/MarshalerUtil.java	0.00%	23 Missing ⚠️
...try/exporter/internal/marshal/ProtoSerializer.java	0.00%	3 Missing ⚠️
...etry/exporter/internal/marshal/JsonSerializer.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6565      +/-   ##
============================================
- Coverage     90.59%   90.31%   -0.29%     
- Complexity     6255     6261       +6     
============================================
  Files           689      690       +1     
  Lines         18698    18762      +64     
  Branches       1843     1853      +10     
============================================
+ Hits          16940    16945       +5     
- Misses         1201     1258      +57     
- Partials        557      559       +2     

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[jack-berg]

Just a couple of minor comments, but looks pretty good!