fix: fix xline member add bug

[Phoenix500526]

Please briefly answer these questions:

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

  1. close issue #661

• what changes does this pull request make?

  1. enrich the cluster_validation test case to cover the whole member add process.
  2. sort the self_peer_urls to match the member.peer_urls and add client URLs
  3. add a single node cluster mode flag to specify a special case —— scale out from one node cluster to a two nodes clsuter.

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 75.60%. Comparing base (e35b35a) to head (2c9504c). Report is 92 commits behind head on master.

Files	Patch %	Lines
crates/curp/src/members.rs	91.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #658 +/- ## ========================================== + Coverage 75.55% 75.60% +0.04% ========================================== Files 180 186 +6 Lines 26938 27488 +550 Branches 26938 27488 +550 ========================================== + Hits 20353 20782 +429 - Misses 5366 5431 +65 - Partials 1219 1275 +56 ```

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[mergify[bot]]

@Phoenix500526 Your PR is in conflict and cannot be merged.

[mergify[bot]]

@Phoenix500526 Convert your pr to draft since CI failed

[mergify[bot]]

@Phoenix500526 Convert your pr to draft since CI failed

[mergify[bot]]

@Phoenix500526 Your PR is in conflict and cannot be merged.

[mergify[bot]]

@Phoenix500526 Your PR is in conflict and cannot be merged.