Make predicate a required flag in attest commands

[lcarva]

Summary

The default value for the predicate parameter for the attest and attest-blob commands is an empty string. However, an empty string is not a valid default value for this parameter resulting in an error.

In #2937, it was suggested that the default value for this parameter should be - (read from stdin). But this results in somewhat odd behavior when stdin is not used.

This commit retains the default value of "", but adds the following guardrails:

1. Makes the --predicate parameter required.
2. Performs an early check for empty string, and reports an error accordingly.

I also ported the new behavior to the attest-blob command.

Release Note

• attest-blob can now read predicate payload from standard input.

Documentation

Updated the CLI docs accordingly.

[codecov]

Codecov Report

Merging #3033 (f54b680) into main (83c08ce) will increase coverage by 0.62%.
The diff coverage is 37.50%.

@@            Coverage Diff             @@
##             main    #3033      +/-   ##
==========================================
+ Coverage   30.45%   31.07%   +0.62%     
==========================================
  Files         152      155       +3     
  Lines        9586     9732     +146     
==========================================
+ Hits         2919     3024     +105     
- Misses       6210     6247      +37     
- Partials      457      461       +4     

Impacted Files	Coverage Δ
cmd/cosign/cli/attest.go	0.00% <0.00%> (ø)
cmd/cosign/cli/attest/attest.go	0.00% <0.00%> (ø)
cmd/cosign/cli/attest_blob.go	0.00% <0.00%> (ø)
cmd/cosign/cli/options/predicate.go	0.00% <0.00%> (ø)
cmd/cosign/cli/attest/attest_blob.go	29.11% <20.00%> (-1.02%)	⬇️
cmd/cosign/cli/attest/common.go	100.00% <100.00%> (ø)

... and 13 files with indirect coverage changes

[znewman01]

lgtm modulo docs

[lcarva]

@znewman01, fixed the docs and a minor issue on the tests 🙏