feat(internal/client/db): introduce `blockchainDB` and the interfaces it implements

[timwu20]

Changes

• Introduces blockchain.Backend interface which embeds blockchain.HeaderBackend and blockchain.HeaderMetadata interfaces. blockchainDB implements blockchain.Backend, and will be used as dependancy in later work inside the introduced db package.
• blockchainDB is essentially the persistance layer for blocks. It will persist to the underlying database.Database, and also provides caching mechanisms to improve performance.
• blockchainDB has a number of subdependencies that are included in this PR.

blockchainDB Dependencies

api.LeafSet

• Manages a list of leaves (leaf blocks) in memory for faster access when reading and modifying active leaves.
• Currently in a client/api package based on location of analagous type in substrate. Will consider moving this within client/db namespace once determined that no future work will need this.

db.pinnedBlocksCache

• Manages pinning of blocks in a fixed size LRU cache. Entries are reference counted along with their justifications and block body.

blockchain.HeaderMetadataCache

• Manages caching of CachedHeaderMetadata types in a fixed size LRU cache.
• Currently in a primitives/blockchain package based on location of analagous type in substrate. Will consider moving this within client/db namespace once determined that no future work will need this.

Primitives

database.Database

• database.Database is the interface to commit transactions as well as retrieve values.
• References the database.Transaction interface which defines a series of changes to the database that committed atomically.
• Used extensively in client/db package.

runtime

• introduces a runtime package which contains interfaces regarding Number, Hasher, Header, Block, Extrinsic, Justification, and Digest.
• a subpackage of within runtime called generic implement block the Block and Header types that conform to the same SCALE encoding as substrate.

Tests

go test -tags integration github.com/ChainSafe/gossamer/internal/

Issues

• closes #3983

[jimjbrettj]

Reviewed about half of it and will continue reviewing it tomorrow

[timwu20]

@P1sar I'm going to resolve our conversations so I can merge this PR. If you have any more questions/comments we can continue on the next PR or schedule a call.