impl: url encode routing keys

[dbolduc]

Part of the work for #12232

I am not adding any tests. I am almost certain that these keys cannot contain characters that need to be escaped. Note that there are no corresponding changes to any generated code...

But it is simple enough to encode them and the encoding is done at code generation time.... So whatever.

---

This change is 

[codecov]

Codecov Report

Patch coverage: 60.00% and project coverage change: -0.01% ⚠️

Comparison is base (56b220f) 93.62% compared to head (e6e6eed) 93.62%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12440      +/-   ##
==========================================
- Coverage   93.62%   93.62%   -0.01%     
==========================================
  Files        2035     2035              
  Lines      180223   180223              
==========================================
- Hits       168742   168741       -1     
- Misses      11481    11482       +1     

Files Changed	Coverage Δ
generator/internal/metadata_decorator_generator.cc	34.54% <0.00%> (ø)
...ator/internal/metadata_decorator_rest_generator.cc	0.00% <0.00%> (ø)
generator/internal/http_option_utils.cc	92.41% <100.00%> (ø)

... and 3 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[coryan]

I think I remember reading that while we currently don't have cases that need this encoding, in the future we might.

This seems like a unit testable thing, if nothing else as a regression test.

[dbolduc]

This seems like a unit testable thing, if nothing else as a regression test.

I was overcome by laziness. Sorry. Still I think I am just going to merge this thing because it seems likely to conflict with other incoming changes.

Most of our unit tests involve parsing protos. I doubt field names can contain characters that would be url escaped... So I doubt that the protos could be parsed.... But admittedly, I didn't try.