add optional arbitrary impls

[tzemanovic]

Description

This adds Arbitrary impl for some types behind a newly added feature "arbitrary" that can be used for e.g. prop and fuzz testing

---

PR author checklist:

• Added changelog entry, using unclog.
• Added tests.
• Linked to GitHub issue.
• Updated code comments and documentation (e.g., docs/).
• Tagged one reviewer who will be the one responsible for shepherding this PR.

Reviewer checklist:

• Reviewed Files changed in the GitHub PR explorer.
• Manually tested (in case integration/unit/mock tests are absent).

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 88 lines in your changes missing coverage. Please review.

Project coverage is 67.08%. Comparing base (edb36f7) to head (bfe33c3).

Files with missing lines	Patch %	Lines
ibc-primitives/src/types/timestamp.rs	0.00%	11 Missing ⚠️
ibc-primitives/src/lib.rs	0.00%	7 Missing ⚠️
ibc-apps/ics20-transfer/types/src/amount.rs	0.00%	5 Missing ⚠️
ibc-apps/ics20-transfer/types/src/denom.rs	0.00%	4 Missing ⚠️
ibc-apps/ics721-nft-transfer/types/src/class.rs	0.00%	4 Missing ⚠️
ibc-apps/ics721-nft-transfer/types/src/lib.rs	0.00%	4 Missing ⚠️
ibc-apps/ics721-nft-transfer/types/src/token.rs	0.00%	4 Missing ⚠️
ibc-core/ics04-channel/types/src/msgs/mod.rs	0.00%	2 Missing ⚠️
ibc-core/ics23-commitment/types/src/commitment.rs	0.00%	2 Missing ⚠️
ibc-apps/ics20-transfer/types/src/coin.rs	0.00%	1 Missing ⚠️
... and 44 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1390      +/-   ##
==========================================
- Coverage   67.35%   67.08%   -0.27%     
==========================================
  Files         225      226       +1     
  Lines       22393    22481      +88     
==========================================
  Hits        15082    15082              
- Misses       7311     7399      +88     

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