Fixed reqwest json issue

[vkomenda]

Problem

error[E0599]: no method named `json` found for type `reqwest::blocking::request::RequestBuilder` in the current scope
  --> watchtower/src/notifier.rs:50:57
   |
50 |             if let Err(err) = self.client.post(webhook).json(&data).send() {
   |                                                         ^^^^ method not found in `reqwest::blocking::request::RequestBuilder`

Summary of Changes

Added the json feature to reqwest in Cargo.toml.

[codecov]

Codecov Report

Merging #8470 into v0.23 will decrease coverage by <.1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           v0.23   #8470     +/-   ##
=======================================
- Coverage   80.6%   80.6%   -0.1%     
=======================================
  Files        251     251             
  Lines      55273   55273             
=======================================
- Hits       44588   44586      -2     
- Misses     10685   10687      +2

[mvines]

Wow I'm surprised that CI didn't catch this issue. I see why now, due to a crazy cargo feature interaction between multiple crates in the repository.

We'll want to make this change on the master branch first, and I see a couple other related fixes that are needed.

[mvines]

Obsoleted by #8475