chore: add Spotless and Prettier action, format files

[ThirdEyeSqueegee]

Issue #, if available:
N/A

Description of changes:

• Add Prettier action for .md and .yml files
• Add Spotless action for .java files (palantir-java-format)
• Add formatting instructions to CONTRIBUTING.md
• Remove push triggers from workflows
• Format files
• Add rutajdash/prettier-cli-action to actions allowlist

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.58333% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.73%. Comparing base (2d42f76) to head (da377b6).
⚠️ Report is 12 commits behind head on v2.

Files with missing lines	Patch %	Lines
.../amazonaws/secretsmanager/caching/SecretCache.java	81.81%	2 Missing ⚠️
.../secretsmanager/caching/cache/SecretCacheItem.java	88.88%	1 Missing and 1 partial ⚠️
...ecretsmanager/caching/cache/SecretCacheObject.java	91.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                 v2     #157      +/-   ##
============================================
+ Coverage     91.40%   91.73%   +0.33%     
- Complexity       87       91       +4     
============================================
  Files             6        6              
  Lines           221      242      +21     
  Branches         24       24              
============================================
+ Hits            202      222      +20     
- Misses           16       18       +2     
+ Partials          3        2       -1     

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