feat: support project level `rate_limit`

[rnbguy]

Closes: #790

With this PR, we should be able to include this in a workspace Cargo.toml:

[workspace.metadata.release]
rate-limit = { existing-packages = 50 }

This would allow a maximum of 50 crates to be published together. This is currently hard-coded to be 30.

[epage]

Could you clean up your commit to being atomic?

[coveralls]

Pull Request Test Coverage Report for Build 9535930480

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 4 of 26 (15.38%) changed or added relevant lines in 4 files are covered.
• 103 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.2%) to 9.065%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/steps/publish.rs	0	3	0.0%
src/steps/release.rs	0	3	0.0%
src/steps/mod.rs	0	4	0.0%
src/config.rs	4	16	25.0%
<!--	Total:	4	26	15.38%	-->

Files with Coverage Reduction	New Missed Lines	%
src/ops/replace.rs	1	0.0%
src/config.rs	4	26.95%
src/ops/shell.rs	21	0.0%
src/steps/changes.rs	77	0.0%
<!--	Total:	103		-->

Totals
Change from base Build 8940883685:	0.2%
Covered Lines:	222
Relevant Lines:	2449

---

💛 - Coveralls

[epage]

Thanks!