feat: Add support for serde_json::Value

[larry0x]

Implement BorshSerialize and BorshDeserialize for serde_json::Value.

This is useful, for example, if a program is to take an arbitrary JSON as input (as Value), and store it as raw bytes.

We found that for this use case, borsh produces much more compact bytes, is faster, and results in smaller program size, compared to using JSON strings.

Thank you!

[larry0x]

Some workflows fail, but they don't seem to be related to my code...

[dj8yfo]

@larry0x i've considered adding this to lib and complementing it with BorshSchema impl
(#314),

but than i took a closer look at serde_json features

vs ascii crate features, support for which has been accepted earlier

which made me realize, that serde_json::Value really qualifies for a high-level type
versus relatively straightforward low-level types from ascii crate,
as e.g. serde_json has arbitrary_precision/raw_value non-default features, which
change the way Value is serialized and affect corresoponding borsh impl-s too.

Therefore i've decided to alter pr to bridge BorshSerialize/BorshDeserialize support to 3rd-party
serde_json::Value in context of a specific application with specific selected serde_json features,
rather than provide limited support to default-only serde_json::Value in the library without really taking into
account any of its intricacies.

Line wise implementation for 3rd party isn't much longer than change of this pr, notable differences being:
https://github.com/dj8yfo/borsh-rs/blob/serde_json_value_examples/borsh/examples/serde_json_value.rs#L72-L81
https://github.com/dj8yfo/borsh-rs/blob/serde_json_value_examples/borsh/examples/serde_json_value.rs#L199-L213
https://github.com/dj8yfo/borsh-rs/blob/serde_json_value_examples/borsh/examples/serde_json_value.rs#L108-L111
https://github.com/dj8yfo/borsh-rs/blob/serde_json_value_examples/borsh/examples/serde_json_value.rs#L182-L191

Also i've commemorated your code in borsh's crate runnable examples and linked to it from README and documentation.

suggested #317 as an alternative instead of this pr

[frol]

Closing in favor of #317