spl: adding metadata account type

[0xC0A1]

Allows for proper usage of MetadataAccounts.

Example

#[account(
        seeds = [b"metadata", mpl_token_metadata::ID.as_ref(), mint.key().as_ref()],
        seeds::program = mpl_token_metadata::ID,
        bump,
        constraint = nft.uses.as_ref().map_or(false, |u| u.remaining > 0) @ AppError::NFTHasNoUses
)]
pub nft: Account<'info, MetadataAccount>

[vercel]

@kevinrodriguez-io is attempting to deploy a commit to the 200ms Team on Vercel.

A member of the Team first needs to authorize it.

[0xC0A1]

@armaniferrante @kklas Do you guys know what is that unrelated error in Tests / Core Tests?
Also the PR seem to be ready as per the new spec right @samuelvanderwaal ?

[kklas]

Seems like you have a linter error.

Run the following in the repo root directory

$ cargo fmt -- --check
$ cargo clippy --all-targets -- -D warnings
$ cargo test

[0xC0A1]

@armaniferrante @kklas @kespinola @samuelvanderwaal Informally re-requesting review because the re-request button isn't working.

[callensm]

@kevinrodriguez-io rebase this branch from master to pull in the clippy error fixes and rerun the tests.