fix: populate default values for contract_metadata

[mitinarseny]

Add #[darling(default)] attribute for link and version to make it possible to derive them from env:

#[near(contract_metadata(standard(standard = "nep141", version = "1.0.0")))
struct Contract {}

As current implementation returns an error:

error: expected an expression
 │   --> src/lib.rs:49:1
 │    |
 │ 49 | / #[near(
 │ 50 | |     contract_state,
 │ 51 | |     contract_metadata(standard(standard = "nep141", version = "1.0.0"))
 │ 52 | | )]
 │    | |__^
 │    |
 │    = note: this error originates in the attribute macro `near` (in Nightly builds, run with -Z macro-backtrace for more info)
 │ 
 │ error[E0425]: cannot find value `CONTRACT_SOURCE_METADATA` in this scope
 │   -->src/lib.rs:49:1
 │    |
 │ 49 | / #[near(
 │ 50 | |     contract_state,
 │ 51 | |     contract_metadata(standard(standard = "nep141", version = "1.0.0"))
 │ 52 | | )]
 │    | |__^ not found in this scope
 │    |
 │    = note: this error originates in the attribute macro `::near_sdk::near_bindgen` (in Nightly builds, run with -Z macro-backtrace for more info)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.03%. Comparing base (936ae63) to head (8bb0041).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1249   +/-   ##
=======================================
  Coverage   80.03%   80.03%           
=======================================
  Files         102      102           
  Lines       14597    14597           
=======================================
  Hits        11683    11683           
  Misses       2914     2914           

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