Fixing interface of axios instance

[pntgupta]

Issue -

• Axios instance returns a function which can be called directly, but the signature to call it is not provided in AxiosInstance interface.

Ex:
let $http = axios.create({...});
$http({...}) //Gives error in typescript (Cannot invoke an expression whose type lacks a call signature. Type 'AxiosInstance' has no compatible call signatures.)

Fix -

• Added function signature in AxiosInstance interface

[k1r0s]

Just read this. You're right but since AxiosStatic extends from AxiosInstance, does it make sense to keep function signatures on AxiosStatic?

981426e from #1244

[pntgupta]

Thanks, missed that

[nickuraltsev]

Thank you for the PR!