feat(prospective-parachains): handleCandidateBacked signal

[viniciussiqueir4]

Changes

• implement HandleCandidateBacked
• create a test: TestHandleBacked

Tests

go test -timeout 10m -run ^TestHandleBacked$ github.com/ChainSafe/gossamer/dot/parachain/prospective-parachains --tags=integration -v

Issues

closes #4309

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✅ rebonat0
❌ viniciussiqueir4
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[EclesioMeloJunior]

@viniciussiqueir4 can you sign the cla, plz?