fix TokenFileWebIdentityCredentials typing

[ChenNima]

Change the typing of the constructor param for TokenFileWebIdentityCredentials to the same as STS.Types.ClientConfiguration, since the whole config is passed down to the STS client.

Fix the issue that type error occurs when changing the STS endpoint via the constructor param of TokenFileWebIdentityCredentials. For example:

new AWS.TokenFileWebIdentityCredentials({
  endpoint: 'https://testendpoint.com'
})

will throw error

Argument of type '{ endpoint: string; }' is not assignable to parameter of type 'ConfigurationOptions'.
  Object literal may only specify known properties, and 'endpoint' does not exist in type 'ConfigurationOptions'.ts(2345)

but actually, the code will change the STS client endpoint successfully.

Checklist

• npm run test passes
• .d.ts file is updated
• changelog is added, npm run add-change