feat(wasm): add Calendly Wasm FDW

[burmecia]

What kind of change does this PR introduce?

This PR is to add a new Calendly Wasm foreign data wrapper.

What is the current behavior?

N/A

What is the new behavior?

N/A

Additional context

N/A

[imor]

Consider using LazyLock<T> instead of unsafe initialization for the INSTANCE static.

[burmecia]

Wasm doesn't have thread support and this instance is only executed in Wasm runtime, so I think the extra locking mechanism may not necessary.

[imor]

If locking is not needed then would LazyCell<T> work? The point was to avoid using unsafe if possible.

[burmecia]

The use of unsafe here is to get/set the instance pointer, so we can easily to use this (aka self) to reference the instance. And the reason to use instance pointer is because this is a Wasm Components Model module, it will be compiled and generated binding code like this:

                pub trait Guest {
                    /// ----------------------------------------------
                    /// foreign data wrapper interface functions
                    /// ----------------------------------------------
                    /// define host version requirement, e.g, "^1.2.3"
                    fn host_version_requirement() -> _rt::String;
                    /// fdw initialization
                    fn init(ctx: &Context) -> FdwResult;
                    /// data scan
                    fn begin_scan(ctx: &Context) -> FdwResult;
                    fn iter_scan(ctx: &Context, row: &Row) -> Result<Option<u32>, FdwError>;
                    fn re_scan(ctx: &Context) -> FdwResult;
                    fn end_scan(ctx: &Context) -> FdwResult;
                    /// data modify
                    fn begin_modify(ctx: &Context) -> FdwResult;
                    fn insert(ctx: &Context, row: &Row) -> FdwResult;
                    fn update(ctx: &Context, rowid: Cell, new_row: &Row) -> FdwResult;
                    fn delete(ctx: &Context, rowid: Cell) -> FdwResult;
                    fn end_modify(ctx: &Context) -> FdwResult;
                }

Our fdw struct will implement that trait, but in those trait functions we cannot access self instance, that's why I have to do a trick to store a static instance pointer as a global variable, and then deref it in the trait functions.

[imor]

It's ok for now @burmecia, I'll maybe try to do this in a separate PR if I find time.