chore: convert OAuth2TokenProvider to Kotlin

package com.fsck.k9.mail.oauth;

package com.fsck.k9.mail.oauth

import com.fsck.k9.mail.AuthenticationFailedException

import com.fsck.k9.mail.AuthenticationFailedException;

public interface OAuth2TokenProvider {

interface OAuth2TokenProvider {

    companion object {

        /**

         * A default timeout value to use when fetching tokens.

         */

    int OAUTH2_TIMEOUT = 30000;

        const val OAUTH2_TIMEOUT: Int = 30000

    }

    /**

     * Fetch a token. No guarantees are provided for validity.

     */

    String getToken(long timeoutMillis) throws AuthenticationFailedException;

    @Throws(AuthenticationFailedException::class)

    fun getToken(timeoutMillis: Long): String

    /**

     * Invalidate the token for this username.

     *

     * <p>

     * Note that the token should always be invalidated on credential failure. However invalidating a token every

     * single time is not recommended.

     * <p>

     *

     * Invalidating a token and then failure with a new token should be treated as a permanent failure.

     */

    void invalidateToken();

    fun invalidateToken()

}