define override directive on fields

[raptros]

relevant to #982
todo

• test locally
• implement the directive correctly (it needs to take a string param)
• update docs

[raptros]

not even sure what version of rust i should try to use....
i tried 1.59 and started getting

± cargo build --all-features
    Updating crates.io index
error: failed to select a version for the requirement `phf = "^0.11"`
candidate versions found which didn't match: 0.10.1, 0.10.0, 0.9.0, ...
location searched: crates.io index
required by package `chrono-tz v0.6.3`
    ... which satisfies dependency `chrono-tz = "^0.6.1"` (locked to 0.6.3) of package `async-graphql v4.0.10 ()`
    ... which satisfies path dependency `async-graphql` (locked to 4.0.10) of package `async-graphql-poem v4.0.10 ()`

which bewilders me

[raptros]

was able to get it to build and test locally ... had to use stable 1.63, and also had to fix some mistakes in my tests

[sunli829]

Thanks! ♥