ESQL: URL encoding changes (elastic#134503)

- Change URL_ENCODE to encode spaces as +.
- Add a new URL_ENCODE_COMPONENT scalar function, which encodes spaces as %20.
- Both encoding functions encode all characters in the input except for alphanumerics, ., -, _, and ~.
- Allow URL_DECODE to fail gracefully if the input can't be decoded, by returning null and adding a warning in the header, similar to other scalar functions.
- Manually perform percent-encoding directly on the BreakingBytesRefBuilder scratch area, and only if the input was gonna change after encoding.
- Update csv-tests:
  - Test decoding for both + and %20 back to space.
  - Reduce the length of doc lines to less than 76 chars, as mentioned in the contribution guide.
  - Add test cases with fixed strings
- Minor changes to the documentation.
- Update Unit tests:
  - Add fixed test cases that fail decoding
  - Randomize the encoder used in decoding tests
  - Use Apache's PercentEncode as the ground truth during unit testing.

Author: mouhc1ne