Change JWT core from being an implicit dependency to explicit one, and bump to v1.87.0.

[Duslerke]

What:

• Make Hackney.Core.JWT be an explicit dependency instead of an implicit one.
• Update the Hackney.Core.JWT package multiple versions up (1.30 -> ... -> 1.87).

Why:

• This C# solution is directly referencing the JWT core package across both its projects: using JWT namespaces, methods, and interfaces. Issue is that this package is not referenced as a direct dependency within the .csproj files. Instead, package is imported through Hackney.Core.Authorization. When the restore happens, Authorization package is build, which contains the binaries from JWT due to a weak link between the 2.
• To get the latest version of the ITokenFactory implementation that adds support for the Cognito token schema.

Notes:

• What changes does latest JWT intoduce? See PR: Add cognito authentication flow token schema support. lbh-core#68.