Add getLedgers implementation

[aditya1702]

What

Closes #111

Adding new endpoint - getLedgers. It has the following request structure:

type GetLedgersRequest struct {
	StartLedger uint32                   `json:"startLedger"`
	Pagination  *LedgerPaginationOptions `json:"pagination,omitempty"`
	Format      string                   `json:"xdrFormat,omitempty"`
}

and the response structure:

// LedgerInfo represents a single ledger in the response.
type LedgerInfo struct {
	Hash            string `json:"hash"`
	Sequence        uint32 `json:"sequence"`
	LedgerCloseTime int64  `json:"ledgerCloseTime,string"`

	LedgerHeader     string          `json:"headerXdr"`
	LedgerHeaderJSON json.RawMessage `json:"headerJson,omitempty"`

	LedgerMetadata     string          `json:"metadataXdr"`
	LedgerMetadataJSON json.RawMessage `json:"metadataJSON,omitempty"`
}

// GetLedgersResponse encapsulates the response structure for getLedgers queries.
type GetLedgersResponse struct {
	Ledgers               []LedgerInfo `json:"ledgers"`
	LatestLedger          uint32       `json:"latestLedger"`
	LatestLedgerCloseTime int64        `json:"latestLedgerCloseTime"`
	OldestLedger          uint32       `json:"oldestLedger"`
	OldestLedgerCloseTime int64        `json:"oldestLedgerCloseTime"`
	Cursor                string       `json:"cursor"`
}

Pagination functionality is similar to getTransactions and getEvents.

Why

This endpoint allows users to access ledger-level info more easily and enables common use-cases: examining ledger sequence numbers, time of close/finality, fees, or getting a snapshot of the ledger at that point in time. For more info refer to the discussion on the original issue ticket.

Benchmarking Details

• BatchGetLedgers: Fetches ledgers in batches from the DB. Note that I have used a batch size of 200 for benchmarking as that is the max ledgers request limit we have set for getLedgers endpoint.

goos: darwin
goarch: arm64
pkg: github.com/stellar/soroban-rpc/cmd/soroban-rpc/internal/db
BenchmarkBatchGetLedgers
BenchmarkBatchGetLedgers-12    	    3655	    337446 ns/op

• GetLedgers: The RPC endpoint to get ledgers.

goos: darwin
goarch: arm64
pkg: github.com/stellar/soroban-rpc/cmd/soroban-rpc/internal/methods
BenchmarkGetLedgers
BenchmarkGetLedgers-12    	    1225	    973161 ns/op

[Shaptic]

Based on this thread, should we rename LedgerCloseMeta -> LedgerMetadata?

[Shaptic]

Should we cherry-pick this onto the protocol22 branch? I don't think it makes sense to target main given how close that is to merging upstream.

[2opremio]

@aditya1702 The PR looks really good! We should resolve the problem of the response ledger range potentially being inconsistent with the ledgers in the response.