Macros: don't reference serde_json::Value directly

[abonander]

The macros can emit code directly referencing serde_json::Value which may trigger compilation errors if the user doesn't have it in their own dependencies: https://discord.com/channels/665528275556106240/665528275556106243/969377568685981716

I'm surprised we haven't encountered this before now, probably because if someone wants to use serde_json::Value in their code deliberately then they probably already have serde_json in their dependencies.

Easy fix, though:

• Change these uses to pub use: https://github.com/launchbadge/sqlx/blob/master/sqlx-core/src/types/json.rs#L4-L5
• Add JsonValue, JsonRawValue to this reexport: https://github.com/launchbadge/sqlx/blob/master/sqlx-core/src/types/mod.rs#L86
• Change serde_json::Value to sqlx::types::JsonValue in the following places:
  • https://github.com/launchbadge/sqlx/blob/master/sqlx-macros/src/database/postgres.rs#L72
  • https://github.com/launchbadge/sqlx/blob/master/sqlx-macros/src/database/postgres.rs#L131
  • https://github.com/launchbadge/sqlx/blob/master/sqlx-macros/src/database/mysql.rs#L54