Updates to Collection and TokenMetadata: Derive attributes

[vickygos]

• In Collection, make init field attributes_frequency_counts optional and if not provided, we fill derive frequency from the collection's tokens' metadata.
• Added a helper method to TokenMetadata to more easily construct metadata, e.g. instead of having to:

# Original way:
TokenMetadata(
      string_attributes={
          "hat": StringAttribute(name="hat", value="blue"),
          "shirt": StringAttribute(name="shirt", value="red"),
      },
      numeric_attributes={
          "level": NumericAttribute(name="level", value=1),
      },
  ),

New optional way:

TokenMetadata.from_attributes(
      {
          "hat": "blue",
          "shirt": "red",
          "level": 1,
      }
  )

• Additionally, added another factory method for Token:
  Before:

Token(
    token_identifier=EVMContractTokenIdentifier(
        contract_address="0xa3049...", token_id=1
    ),
    token_standard=TokenStandard.ERC721,
    metadata=TokenMetadata(
        string_attributes={
            "hat": StringAttribute(name="hat", value="cap"),
            "shirt": StringAttribute(name="shirt", value="blue"),
        }
    )

Now you can do:

Token.from_erc721(
    contract_address="0xa3049...",
    token_id=1,
    metadata_dict={"hat": "cap", "shirt": "blue"},
)