Add typing: pair

[krzema12]

Some actions accept a list of key-value pairs.

Examples:

https://github.com/aws-actions/amazon-ecs-render-task-definition/blob/ac1938bb1280b4d5564fdefc715af5d15f7f1c57/action.yml#L16-L18

https://github.com/aws-actions/amazon-ecs-render-task-definition/blob/ac1938bb1280b4d5564fdefc715af5d15f7f1c57/action.yml#L22-L24

https://github.com/aws-actions/amazon-ecs-render-task-definition/blob/ac1938bb1280b4d5564fdefc715af5d15f7f1c57/action.yml#L25-L27

https://github.com/madhead/intellij-http-client-action/blob/cfc73485b84b8dc42cdc6b8fe5977e4232c98d52/action.yml#L30-L32

https://github.com/docker/metadata-action/blob/31cebacef4805868f9ce9a0cb03ee36c32df2ac4/action.yml#L17-L19

We could add support for these, to avoid describing them as list of strings.

E.g. in Kotlin, it would be mapped to List<Pair>, and would be possible to write listOf("foo" to "bar", "baz" to "goo").

Consider (also?) introducing a dictionary typing, maybe it would cover this case in a more correct way.