feat(api/client): add support for multiple endpoints with failover

[DeVikingMark]

Implements support for multiple endpoints in celestia client library to provide failover capabilities for both Bridge DA and Core gRPC connections.

Changes

• Add AdditionalBridgeDAAddrs field to ReadConfig for multiple bridge endpoints
• Add AdditionalCoreGRPCConfigs field to CoreGRPCConfig for multiple core endpoints
• Introduce MultiClient with NewMultiEndpoint() constructor
• Add validation for additional endpoint configurations
• Update documentation and examples

Usage

cfg := client.Config{
    ReadConfig: client.ReadConfig{
        BridgeDAAddr: "http://primary:26658",
        AdditionalBridgeDAAddrs: []string{"http://backup:26658"},
    },
    SubmitConfig: client.SubmitConfig{
        CoreGRPCConfig: client.CoreGRPCConfig{
            Addr: "primary:9090",
            AdditionalCoreGRPCConfigs: []client.CoreGRPCConfig{
                {Addr: "backup:9090", TLSEnabled: true},
            },
        },
    },
}

multiClient, err := client.NewMultiEndpoint(ctx, cfg, kr)

Fixes #4487

[codecov-commenter]

Codecov Report

❌ Patch coverage is 33.33333% with 122 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.26%. Comparing base (2469e7a) to head (3ec4c7b).
⚠️ Report is 592 commits behind head on main.

Files with missing lines	Patch %	Lines
api/client/client.go	27.86%	43 Missing and 1 partial ⚠️
api/client/grpc.go	17.77%	37 Missing ⚠️
api/client/example/example.go	0.00%	28 Missing ⚠️
api/client/read_client.go	73.46%	9 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4617      +/-   ##
==========================================
- Coverage   44.83%   35.26%   -9.58%     
==========================================
  Files         265      304      +39     
  Lines       14620    24504    +9884     
==========================================
+ Hits         6555     8641    +2086     
- Misses       7313    14905    +7592     
- Partials      752      958     +206     

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