ingest/cdp: Create low level helper functions for pulling out specific fields in LCM

[chowbao]

As part of the CDP processor design/brainstorm, we all agreed that there's benefit in creating low level helper functions that users can call to get the specific field/data for their needs.

There is a draft PR that relates to the low level helper functions. Note: placed randomly in stellar-etl for convenience

Things to decide/discuss:

• Where should these helper functions live? New package, ingest, go/xdr?
• Which fields should we target first? (keep initial scope low but enough to be useful)
  • Ideally we could have functions for every field/data
  • Even if we only target ledgers, transactions, and operations the number of fields is still quite large
  • Is it possible to leverage XDR <> JSON functions at all to help this?
• Should we define a standard interface/template for these low level helper functions?
• Should we return go data types or the xdr types like xdr.Uint32?
  • I'd lean go types
• What should happen if there are similarly named fields?
  • Low code function to return account - which account (buyer/seller)

Examples and cases to consider:
Simple case: easy traversing through LCM and returning a simple type

func BaseFee(lcm xdr.LedgerCloseMeta) (uint32, error) {
	ledgerHeader := lcm.LedgerHeaderHistoryEntry()
	baseFee := ledgerHeader.Header.BaseFee
	return uint32(baseFee), nil
}

Cases with V# xdr structs

func SorobanFeeWrite1Kb(lcm xdr.LedgerCloseMeta) (int64, error) {
	switch lcm.V {
	case 0:
		return nil, nil
	case 1:
		lcmV1Ext := lcm.MustV1().Ext
		switch lcmV1Ext.V {
		case 0:
			return nil, nil
		case 1:
			ext := lcmV1Ext.MustV1()
			sorobanFreeWrite1Kb := ext.SorobanFeeWrite1Kb
			return int64(sorobanFreeWrite1Kb), nil
		default:
			panic(fmt.Errorf("unsupported LedgerCloseMeta.V1.Ext.V: %d", lcmV1Ext.V))
		}
	default:
		panic(fmt.Errorf("unsupported LedgerCloseMeta.V: %d", lcm.V))
	}
}

Cases where the return type could be more subjective like with xdr.ScVal

relates to: #5476
blocks: #5477, https://stellarorg.atlassian.net/browse/HUBBLE-231