refactor(otlp-exporter): move code that will be removed to legacy directory

[pichlermarc]

NOTE: draft PR, not ready for review

Which problem is this PR solving?

See #4415 for a more complete picture. This PR moves all existing code that will eventually be removed to a legacy subdirectory.

Reason: to keep the diff for PRs manageable we won't immediately transition every exporter to use the new implementation. As we don't transition immediately, we need to keep the legacy code around for a while until we can remove it.

Type of change

Please delete options that are not relevant.

• refactor (does not change the public interface, keeps all functionality as-is)

How Has This Been Tested?

• Existing unit tests

[codecov]

Codecov Report

Merging #4449 (bdf4016) into main (4655895) will decrease coverage by 0.26%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4449      +/-   ##
==========================================
- Coverage   92.23%   91.98%   -0.26%     
==========================================
  Files         336      341       +5     
  Lines        9533     9593      +60     
  Branches     2022     2029       +7     
==========================================
+ Hits         8793     8824      +31     
- Misses        740      769      +29     

Files	Coverage Δ
...etrics-otlp-proto/src/legacy/OTLPMetricExporter.ts	100.00% <100.00%> (ø)
.../otlp-exporter-base/src/legacy/OTLPExporterBase.ts	95.23% <ø> (ø)
...al/packages/otlp-exporter-base/src/legacy/index.ts	100.00% <100.00%> (ø)
...legacy/platform/browser/OTLPExporterBrowserBase.ts	17.14% <ø> (ø)
...exporter-base/src/legacy/platform/browser/index.ts	100.00% <ø> (ø)
...-exporter-base/src/legacy/platform/browser/util.ts	34.84% <ø> (ø)
...es/otlp-exporter-base/src/legacy/platform/index.ts	100.00% <ø> (ø)
...e/src/legacy/platform/node/OTLPExporterNodeBase.ts	50.00% <ø> (ø)
...lp-exporter-base/src/legacy/platform/node/index.ts	100.00% <ø> (ø)
...lp-exporter-base/src/legacy/platform/node/types.ts	100.00% <ø> (ø)
... and 3 more