Adding throughput field for volumes in provider spec

[rishabh-11]

What this PR does / why we need it:
This PR makes it possible to configure throughput for different volume types. As of today it is configurable only for gp3 volume type and will return an error from the provider side if specified for any other volume type.

Which issue(s) this PR fixes:
Fixes #93

Special notes for your reviewer:
The following tests were performed on a cluster in the dev landscape:- (Current range of throughput is 125MiB/s - 1000MiB/s)
(The throughput field is configured in machine class yaml and then a machine is deleted to trigger the creation of new machines. The machine controllers are run locally after scaling down the mcm running in the control plane of the dev cluster.)

1. Throughput=-100, Volume Type = {gp3, io1, gp2} --> Error is returned with the following message:- Invalid value: -100: throughput must be a positive value
2. Throughput=300, Volume Type = {gp2, io1} --> Error message is as follows:- InvalidBlockDeviceMapping: throughput cannot be specified with the volumeType of device '/dev/xvda'\n\tstatus code: 400
3. Throughput=300, IOPS=3500, Volume Type = gp3 --> Machine with throughput=300 and IOPS=3500 is created.
4. Throughput=1001, IOPS=3500, Volume Type = gp3 --> Error message is as follows:- InvalidParameterValue: Volume throughput of 1001 is too high; maximum is 1000.\n\tstatus code: 400
5. Throughput=100, IOPS=3500, Volume Type = gp3 --> machine with throughput=125 and IOPS = 3500 is created.

In all the above cases where an error is returned, it will be shown as part of the machine status in the machine yaml. The state of the machine will be CrashloopBackoff.

Release note:

Throughput is now configurable for volume types. Its validation i.e. whether it is allowed or not for the particular volume type and is within the range, is done on the provider(AWS) side. Currently only gp3 volume have configurable throughput.

[gardener-robot]

@rishabh-11 Thank you for your contribution.

[himanshu-kun]

/lgtm