Tokens Subproject

[couralex6]

Introducing a new DBT project to Spellbook. As announced here, we're breaking our massive DBT project into several smaller ones. The first subproject to be spun out is Tokens.

This PR introduces the new subproject as well as some refactoring to enable a multi DBT project repo.

Couple things to note:

• We introduce a new directory sources/ that will contain all source definitions (*_sources.yml). The Tokens subproject (and future ones) will read sources from this directory. This prevents us from having to duplicate source definitions across different subprojects. This means we will have to define sources in this location from now on.
• The macros/ directory will also be shared across subprojects. This also allows us to avoid macro duplication. This means that we will be able to use all the existing macros in future subprojects without having to duplicate the file.
• No cross project ref(). Inn order to read from a spell in tokens, we will have to use the source() function.

This is the list of models moved to tokens subproject:

models/aave/aave_v3_tokens.sql
models/ovm/optimism/ovm_optimism_l2_token_factory.sql
models/the_granary/optimism/the_granary_optimism_tokens.sql
models/tokens/arbitrum/tokens_arbitrum_erc20.sql
models/tokens/avalanche_c/tokens_avalanche_c_erc20.sql
models/tokens/base/tokens_base_erc20.sql
models/tokens/bnb/tokens_bnb_bep20.sql
models/tokens/celo/tokens_celo_erc20.sql
models/tokens/tokens_erc20.sql
models/tokens/ethereum/tokens_ethereum_erc20.sql
models/tokens/fantom/tokens_fantom_erc20.sql
models/tokens/gnosis/tokens_gnosis_erc20.sql
models/tokens/goerli/tokens_goerli_erc20.sql
models/tokens/tokens_native.sql
models/tokens/optimism/tokens_optimism_erc20.sql
models/tokens/optimism/tokens_optimism_erc20_bridged_mapping.sql
models/tokens/optimism/tokens_optimism_erc20_curated.sql
models/tokens/optimism/tokens_optimism_erc20_generated.sql
models/tokens/optimism/tokens_optimism_erc20_transfer_source.sql
models/tokens/polygon/tokens_polygon_erc20.sql
models/tokens/zksync/tokens_zksync_erc20.sql
models/tokens/zora/tokens_zora_erc20.sql
models/yearn/optimism/yearn_optimism_vaults.sql