This patch rewrites how the HTTP credentials provider works to allow
both ECS and EKS identities to work. It is based on the aws-sdk-go-v2
implementation.

It validates that the endpoint is correct if the transport is HTTP, but
does not support DNS resolution, however based on how the pod identity
agent works today, DNS should not be needed. If the transport is HTTPS,
which will not be the case when using EKS Pod Identities, any endpoint
is allowed. This is in line with how the AWS SDK works.

Similarly to the SDK, it also reads the authentication token environment
variables, with the file taking precedence over the raw token variable.

This has been tested against an EKS 1.30 cluster with AL2023 nodes.

Signed-off-by: Andrew Titmuss <iandrewt@icloud.com>

This patch fixes the handling of IPv6 literals, which would fail to
parse after not finding balanced `[]` characters.

This happened because it searched for the first `:` before copying the
host portion, so (assuming it didn't return NULL), the endpoint
`[fd00:ec2::23]` would become `fd00`.

I've added some tests around this case, but I wouldn't describe the
tests as in-depth.

This patch is needed for EKS Pod Identities to work on IPv6 clusters.

Signed-off-by: Andrew Titmuss <iandrewt@icloud.com>

This error handler is for JSON APIs that respond with a Code and Message
field in their error responses.

It is originally from PettitWesley/fluent-bit@7a2c1a8

Signed-off-by: Andrew Titmuss <iandrewt@icloud.com>

This patch fixes the following:
1. ensures malformed tokens (containing `\r\n`) are treated as invalid
2. fixes allowing any https host, which previously went through the same
   checks as http hosts
3. fixes a memory leak when provider initialisation fails
4. logs errors from the HTTP agent
5. rewrites the tests using the cases from PettitWesley/fluent-bit@195db30

Signed-off-by: Andrew Titmuss <iandrewt@icloud.com>